The presence of current gas predicted by our model in our Solar System would not impact the dynamics of bodies (dust or planetesimals) evolving around the KB. However, we note that in the past, when the Kuiper belt was much younger and heavier, the release of CO would have been orders of magnitude larger (above the solid line in Fig. REF ) and the gas dynamics would have also been much different (e.g., in the fluid/hydrodynamic regime), potentially leading to some interesting effects, such as delivering some CO mass from the KB to planetary atmospheres as proposed recently for extrasolar systems [1]}. Indeed, in more massive disks, gas becomes optically thick to the SW and it does not get pushed outwards. Instead, gas drifts inwards because of viscous evolution and it can end up accreted onto planets (see Appendix ). The initial KB may have been much more massive before potential dynamical instabilities [2]} and could have led to CO outgassing rate close to the dashed line in Fig. REF , hence providing CO that falls onto the young planets in greater quantity than other potential sources such as impacts [1]}. This is a whole new study that emerges naturally from this work and will be tackled in a different paper.
